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Tuesday, June 7, 2011

Australopithecine teeth: evidence for mating patterns?

A Naturepaper,and a Nature News story about it, widely covered in the news (e.g., here), concerns the mating patterns of 2 million year old Australopithecines (human ancestors, or related side-branch(es)) and their mating patterns, as inferred from mineral analysis of teeth.

Fossilized teeth of early human ancestors bear signs that females left their families when they came of age, whereas males stayed close to home.

A chemical analysis of australopithecine fossils ranging between roughly 1.8 million and 2.2 million years old from two South African caves finds that teeth thought to belong to females are more likely to have incorporated minerals from a distant region during formation than those from males.

In non-human primate societies, it is typical for adolescent males to get into conflict with adult males in the local group or troop, and be kicked out. They then have to break into neighboring groups to find mates. In most 'primitive' human societies, it is the female exogamy that is most common. That is, females must find a mate in a different--usually nearby adjoining--local village or population, often also specified by some sort of social or kinship relationship (clan, type of cousin, etc.).

Here's a summary of the interpretation being given to the paper (in part by the authors, but extended greatly in the usual way by the melodramatic media--and, as usual, by Nature). This study of Australopithicene specimens in Southern Africa reports that male teeth all seem to share the same molecular signature of dietary intake when young that is consistent with residence in the local area where they were found. But the females show molecular signatures of distant locations. This suggests to the authors that there was female exogamy at that time. Whether or not these Australopithecines were direct human ancestors, the authors infer that they showed that the typical human mating pattern had begun to evolve.

But hold on. In fact, the samples were small.The inference was made based on current assessment of the morphology of Australopithecine specimens that this was a single, sexually dimorphic species. If that is so, then relatively small teeth would be from females, and larger teeth from males. But of course this is an assummption. Dental molecular composition was compared between small and large teeth, assuming they represented males and females. Other animal fossils were used as comparison, assuming taxa that today have particular ranges had them back then. Molecular evidence in males was that it seemed similar to local-remaining species, and the nature of the environment was measured by molecular data from current species--modern plants and animals within 50 km of the Australos' sites. And the evidence was statistical, that is, 'at least 50%' of the females were non-local by the authors' criteria, while only 11% of the males were. The statistical significance was marginal (p=0.49), and given the kinds of data manipulation one does with such indirect inference, and the prior assumptions made, is rather weak. But the authors then found that there were more non-local small hominins than non-local small-range mammals, with p=0.028 (somewhat more statistically significant) but no such distinction for non-local large hominins and non-local small-range mammals, again suggesting that the small hominins were relatively non-local.

This is interesting but must be viewed as circumstantial and rather weak evidence. An important question is how, in this case, the inference could have been made about population structure since usually not only do the individuals come from nearby demes--not more than 50 km away?--but they keep exchanging every generation, and essentially every, not just some, male or female in a group has to follow the same rules. Also, we must accept that these specimens were essentially contemporary, but of course fossil dating is not so precise as a rule. Unless the environmental patterns varied very locally how could one determine origins? The authors didn't, so far as we see in the paper, identify by molecular means where the females came from--only that their signature was 'non-local'.

Still, this is of some potential interest, This is especially so if the different environments were far apart, to understand how contemporaries--mates--know of each other's group and travel so far to meet each other? Such things, if true, would be interesting to understand! In sparse settlement areas, with hunter-gatherer population densities (in humans ~1 person per 10 square km), distances could be traveled on foot that could exceed 50km, at least in principle for modern hunter-gatherers. So, even if we reject the authors' assumptions, it is interesting to ask what explains their findings. And if we accept them, again, we have interesting things to think about.

Note that these 'people' were not 'people' in the modern sense, as we only came on the scene around 100,000 years ago, not 2 million. And they were not 'cavemen' and 'cavewomen' as the hyperbolic telegenic press so typically categorizes anyone not living in New York or London today.

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